Modern magnetically based electronic article surveillance systems generally derive their parentage from 1934 French Patent No. 763,681. That patent depicts the use of markers formed of a piece of low coercive force, high permeability alloy, such as permalloy, and teaches that when the magnetization of such a piece is reversed by a magnetic field alternating at a fundamental frequency, detectable harmonics of that frequency will be produced.
More recently, various investigators have developed magnetic markers which have dual-status capabilities. Typically, as first disclosed in U.S. Pat. Nos. 3,665,449 (Elder et al.) and 3,747,086 (Peterson), such dual-status markers include at least one piece of low coercive force, high permeability material together with at least one piece of remanently magnetizable material. When the latter piece is magnetized it has associated therewith a magnetic field which biases the low coercive force, high permeability material so as to alter the signal produced when the biased material is in the interrogation field. It is also disclosed in the '449 patent that such dual-status markers may comprise coextensive strips of magnetizable material and high permeability, low coercive force material, and while not preferred, that the magnetizable material could be uniformly magnetized.
Similarly, one marker embodiment depicted in the '086 patent comprises two coextensive strips. While that patent indicates that magnetization of one strip alters the harmonic content of the signal produced by the other, the exact nature of the magnetization is not specified. The disclosure pertaining to FIG. 6D of the '086 patent suggests only that magnetization be such as to leave the responder strip in a fully magnetized condition, thereby causing the marker to be completely silent.
The '449 and '086 patents thus suggest that single directionally responsive markers may be deactivated by a magnetic bias field extending the full length of the responder strip, but fail to enable that suggestion. Rather, by following the teaching in those and subsequent patents it has become well recognized that reliable deactivation is obtained by providing discontinuous fields so that the responder strip essentially responds as a number of strips of shorter length. This is effected in typical, commercially viable systems by providing a number of magnetizable pieces spaced along the responder strip or by providing a continuous strip of magnetizable material which is magnetized in bands of alternating polarity.
More recently, multi-directionally responsive magnetic markers have also been developed. Thus, for example, as set forth in a prior patent of the present inventor, U.S. Pat. No. 4,710,754, such markers may comprise a square piece of low coercive force, high permeability material fabricated to have regions with narrow widths centered along each edge of the squares, thereby providing switching sections, and extensive regions in each corner which collect and channel flux into the switching sections. The markers of the '754 patent are made dual-status by adding discrete pieces of magnetizable material adjacent each switching section.
A further embodiment of a dual-status, multi-dimensionally responsive marker is disclosed in U.S. Pat. No. 4,825,194 (Church et al.) in which discrete magnetizable pieces are positioned adjacent flux collector sections of a sheet of responder material. Optionally, that patent also suggests that additional pieces of magnetizable material may be positioned adjacent the switching sections, but that the separation between the respective magnetizable pieces be sufficient to prevent appreciable magnetic coupling therebetween.
Multi-dimensionally responsive markers in which a coextensive sheet of magnetizable material is provided together with a sheet of low coercive force, high permeability responder material are disclosed in a second patent of the present inventor, U.S. Pat. No. 4,746,908. However, the markers of the '908 patent function in a significantly different manner and utilize a piece of responder material configured so as not to create a desired response. The coextensive sheet of magnetizable material is magnetized with a predetermined pattern which biases only adjacent portions of the responder material, thereby inhibiting response from those portions. The magnetized pattern is such that the dimensions of the unbiased, remaining portion can then produce the desired response. Such markers thus function oppositely to those in typical use, i.e., that the marker is magnetized when in its sensitive state.
A third patent of the present inventor, U.S. Pat. No. 4,967,185, discloses that multi-dimensionally responsive markers somewhat similar to those preferred in the '754 patent may be reliably changed from a first, active state, to a second, deactive state, by applying a magnetic field to uniformly magnetize a coextensive magnetizable sheet in any direction in the plane of the sheet. The marker may be subsequently changed, or switched back to the active state by demagnetizing the magnetizable sheet. Such a marker thus may comprise two coextensive magnetic sheets in which the width of the sheets is not less than one-half the length. The first sheet may be selected of a material having a high permeability and low coercive force, which is configured to have at least two, mutually perpendicular elongated areas proximate to the periphery of the sheet. Each of the elongated areas is capable of responding to an alternating magnetic field in an interrogation zone generally applied along the length of the area to result in the production of an alarm. Each area thus includes a narrow width region forming a switching section and extends on each end along the length into extensive regions forming flux collector sections for the adjacent switching section.
The second sheet is selected of a remanently magnetizable material, which overlies and is magnetically coupled to the sheet of responder material. This magnetizable sheet, when substantially uniformly magnetized in the plane of the sheet, causes alternate polarity switching pulses resulting from a reversal of magnetization of the switching sections when exposed to alternating fields, to be shifted in time and/or altered in amplitude. Markers having the magnetizable sheet alternatively magnetized or demagnetized can then be distinguished from each other.
As noted above, the two states of the marker of the '185 patent are manifested by differences in the time at which alternate polarity pulses are produced and by differences in the amplitude of the respective pulses, depending upon whether or not the magnetizable sheet is magnetized. That patent invention thus also includes an EAS system for use with such markers. In addition to the markers themselves, the system thus comprises means, such as a drive oscillator, amplifier, and field coils, for generating within an interrogation zone an alternating magnetic field, means for receiving marker produced signals and ultimately producing an alarm signal when appropriate and means for magnetizing the magnetizable material in the markers. The magnetizing means preferably provides a single, substantially uniform magnetic dipole in the magnetizable sheet, one edge of the sheet having one magnetic polarity and an opposite edge having the opposite polarity.
The receiving means receives signals resulting from flux changes in the marker produced when the marker is exposed to the alternating field in the zone. Means are also included for distinguishing between signals from the markers when the piece of magnetizable material is either magnetized to have a said single magnetic dipole or is demagnetized, and from other signals as may be caused by ambient effects, random ferromagnetic objects and the like. The distinguishing means further comprises means responsive to differences in the amplitude of marker produced signals and to relative displacements of alternate signal components for producing an alarm signal when appropriate.